In a coating apparatus of the curtain coating type, a moving support is coated by causing a free falling curtain of coating liquid, referred to hereafter as simply the curtain, to impinge on a moving support to form a layer thereon. An apparatus to perform this method is described in U.S. Pat. No. 3,508,947 to Hughes wherein a multilayer composite of a plurality of distinct liquids is formed on a slide hopper and dropped therefrom to form a falling curtain.
In the curtain coating process, particularly as used to manufacture multilayer photographic materials, the quality of the coating is largely determined by the properties of the liquid curtain. It is important to ensure that a stable laminar flow of coating solution is formed by the slide hopper and that an equally stable laminar liquid curtain is formed from that coating solution. To prevent contraction of the edges of the falling curtain under the effect of surface tension it is known that the curtain must be guided at its edges by curtain edge guides.
It is well known in the curtain coating art that introduction of a lubricating liquid between the curtain and the edge guide will improve the operation of the curtain. These improvements include the ability to maintain the curtain at lower total flow rates with lubricating liquid than without, and the ability to maintain curtains of higher viscosity with a lubricating liquid than without. Typically, the lubricating liquid is simply water; however, an alternate liquid of low viscosity may be used for the same purpose.
Surfactants are typically added to the outer layers of the multiple layer curtain coating application. As the layers flow down the hopper slide the surfactant has some time to diffuse from the bulk of the liquid to the surface and lower the surface tension of the top layer. When the lubricating liquid is introduced abruptly its initial surface tension is unavoidably high compared to the curtain even if there is surfactant added since there is no time for surfactant diffusion. This creates a surface-tension-driven flow which moves some of the high viscosity curtain solutions to the edge guide. This increases the effective viscosity near the edge guide and thereby increases wall drag. This increased wall drag reduces the momentum of the edge of the curtain and in turn limits coating speed and compromises the uniformity of the edge thereby causing waste.
The present invention provides a method for issuing a lubricating band of liquid along an edge guide with a surface tension similar to the curtain solution and without disturbance to the curtain. This is accomplished in a very short vertical distance from the lip, thus maximizing the velocity of the curtain solutions near the edge. The lubricating fluid band is also in laminar flow, thus avoiding wavy edge and turbulent wave problems caused by turbulent edge solution flow.